ras Gene Activation and Carcinogenesis. We have identified activating point mutations in ras genes in about 15% of the most common forms of human cancers. By using RFLP assays we have also demonstrated the somatic nature of these activating point mutations. Amplification of otherwise unaltered ras proto-oncogenes may also lead to malignant transformation. We have demonstrated that the combined effect of multiple copies of the human H-ras-1 proto-oncogene induces malignant transformation of NIH/3T3 cells. We have also reported that amplification of ras genes can be observed in human tumors although at a relatively low frequency. These results show that ras gene amplification occurs in unmanipulated tumor biopsies and, therefore, is not a consequence of in vitro establishment of cell lines. Moreover, they indicate that gene amplification is an alternative pathway by which ras can also contribute to neoplastic development. Monoclonal Antibodies Against ras p21 Proteins. Computer analysis of the predicted amino acid sequences of normal and transforming ras p21 proteins indicates the existence of significant structural differences. These observations raised the possibility that monoclonal antibodies may be elicited against the structural domains specific for transforming p21 proteins. These antibodies could be valuable diagnostic reagents and would be useful to study the functional role of ras p21 proteins. The availability of large amounts of highly purified normal and transforming ras p21 synthesized in E. coli has made possible the development of immunization and screening protocols. A panel of positive hybridomas are being currently screened against purified normal and transforming ras p21 proteins for their ability to exhibit differential affinity for either of the two structural forms of p21, and to interact with the known biochemical activities of these proteins.